One of the many ways to improve engine performance in motor vehicles is by means of electronic controlled valve actuation systems. In these systems, an electronic control unit, ECU, receives signals from several different sensors in the vehicle such as speed, load, temperature, etc., and by means of certain algorithms in the electronic control unit, the optimum time to open and close engine valves is calculated. The optimum time is converted to an electronic signal, either digital or analog, and supplied to a solenoid operated control valve to control the flow of hydraulic fluid in and out of a hydraulic tappet or valve lifter.
U.S. Pat. No. 4,615,306 issued to Russell Wakeman and entitled "Engine Valve Timing Control System" describes a basic engine valve control system wherein the opening and closing of the hydraulic valve lifters creates pressure pulses in the hydraulic fluid to provide a boost pulse to move the lifter to its extreme position. An electronic control unit controls the operation of the solenoid operated valve to add or remove hydraulic fluid from the lifter to create a fluid link in the lifter transferring motion from the timing cam on the camshaft to the engine valve.
U.S. Pat. No. 4,223,648 issued to Pozniak et al and entitled "Hydraulic Valve Lifter" describes a "leak-down" hydraulic lifter wherein a Belleville spring valve disc having a thin, sharp edge orifice that is operative to sense the oil flow rate between the valve chamber 53 and the reservoir chamber 24 to regulate the volume capacity of the pressure chamber and therefore the axial extent of the valve lifter as a function of engine speed. Actual leakage of the fluid due to the fit between adjacent telescoping parts permits the lifter to collapse as the tappet approaches the high point of the timing cam. It is well known that in determining flow through an orifice, as the length of the orifice becomes smaller and smaller, the viscosity factor of the fluid in effect disappears from the equation. This was recognized in the '648 patent. This patent is concerned with the collapse of the lifter at the beginning of the valve motor, thus reducing the duration and is not concerned with the closure of the valve. As the engine gains speed, RPM, the valve mechanism will close faster and the duration will become longer. No lifter can extend during valve motion, unless it actually becomes separated from the cam or rocker arm; this is called float.
SAE Technical Paper #840335 presented at the 1984 SAE International Congress & Exposition by the patentees of the '648 patent also discusses in greater detail the aspects of the variable valve timing lifter.